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IC 


8920 



Bureau of Mines Information Circular/1983 




Characteristics of the OTOX 
Model CTL Oxygen Sensor 



By J. E. Chilton, G. H. Schnakenberg, Jr., 
and L. Spinetti 




UNITED STATES DEPARTMENT OF THE INTERIOR 



Information Circular 8920 



Characteristics of the OTOX 
Model CTL Oxygen Sensor 



By J. E. Chilton, G. H. Schnakenberg, Jr., 
and L. Spinetti 




UNITED STATES DEPARTMENT OF THE INTERIOR 
James G. Watt, Secretary 

BUREAU OF MINES 
Robert C. Norton, Director 






This publication has been cataloged as follows: 



Chilton, J. E. 

Characteristics of the OTOX model CTL oxygen sensor. 

(Bureau of Mines information circular ; 8920) 

Includes bibliographical references, 

Supt. of Docs, no.: I 28.27:8920. 

1. Mine gases— Measurement— Equipment and supplies. 2. Oxygen- 
Analysis— Equipment and supplies. 3. Gas-detectors— Testing. 1. 
Schnakenberg, George H. II. Spinetti, L. III. United States. Dept. 
of the Interior. IV. Title. V. Series: Information circular (United 
States. Bureau of Mines) ; 8920. 

XN^e^TtH [TN305] 622s [622'. 8] 82-600364 



CONTENTS 

Page 

Abstract 1 

Introduction 2 

Theory 3 

Experimental systems 4 

Discussion 5 

Stability 5 

Bias over range 7 

Temperature effect 9 

Pressure effect 10 

Pressure surge tests 13 

Response times 14 

Summary 15 

ILLUSTRATIONS 

1 . OTOX oxygen sensor 3 

2. OTOX sensor Initial stability 5 

3. OTOX sensor long-term stability 7 

4. Zero-corrected oxygen response for the OTOX sensor 8 

5. Normalized oxygen response for the OTOX sensor 9 

6 . OTOX sensor temperature response 10 

7. OTOX sensor temperature response for the square root of the absolute 

temperature 11 

8 . OTOX sensor pressure effect 12 

9. OTOX sensor pressure surge effect: Initial pressure +8 In Hg above at- 

mospheric pressure 13 

10. OTOX sensor pressure surge effect: Initial pressure -8 In Hg below atmos- 

pheric pressure 14 

11. Sample time responses for the OTOX sensor 15 

TABLES 

1. Stability of OTOX sensors; sensor response for 20.9 pet O2 6 

2. Linear regression analysis for stability data on OTOX sensors 6 

3 . OTOX sensor oxygea response 8 

4. OTOX sensor normalized oxygen response data 9 

5. Linear regression analysis for OTOX response to oxygen concentration. 9 

6. OTOX sensor temperature response In air 10 

7. Effect of static pressure on sensor response, OTOX CTL 11 

8. Linear regression analysis for OTOX response to change In static pressures 13 

9. Pressure surge response time data , 14 

10. Response times for step change In oxygen concentration to 90 pet of final 

value 14 



UNIT 


OF MEASURE ABBREVIATIONS 


USED IN 


THIS REPORT 


cm 


centimeter 


mL 


milliliter 


cm 


cubic centimeter 


mm 


millimeter 


° C 


degree Celsius 


mV 


millivolt 


g 


gram 


Pa 


pascal 


in 


inch 


pet 


percent 


K 


kelvin 


sec 


second 


min 


minute 







CHARACTERISTICS OF THE OTOX MODEL CTL OXYGEN SENSOR 

By J. E. Chilton, ' G. H. Schnakenberg, Jr., ^ and L. Spinetti -^ 



ABSTRACT 

The Bureau of Mines has examined the operation of an oxygen sensor 
manufactured by the City University, London, England. The sensor pro- 
duces a current proportional to the oxygen concentration by reacting 
oxygen at a nonconsumable cathode. The sensor design is unique in that 
the primary mode of oxygen transport to the cathode is by diffusion 
through a capillary. The sensor using this design has a stability of 
0- to 0.02-pct reading change per day over a 4.7-month test, small tem- 
perature coefficient of 0.29-pct reading change per ° C, and small 
pressure coefficient of 0.34-pct reading change per 1,000-ft altitude 
change. If this sensor were incorporated into an oxygen detector or 
monitor, this would be a distinct improvement in electrochemical-type 
oxygen analyzers and would be useful to the mining industry, 

^Research chemist, Pittsburgh Research Center, Bureau of Mines, Pittsburgh, PA. 

^Supervisory research physicist, Pittsburgh Research Center, Bureau of Mines, 
Pittsburgh, PA. 

•^Chemist, Pittsburgh Research Center, Bureau of Mines, Pittsburgh, PA. (now 
retired) . 



INTRODUCTION 



An oxygen detector is used in mining 
operations for verifying that there is an 
adequate supply of oxygen for life sup- 
port of miners. The Code of Federal Reg- 
ulations requires a supply of air con- 
taining not less than 19.5 vol-pct O2 for 
underground coal mine ventilation (30 CFR 
75.301). To verify that ventilation air 
meets this specification, oxygen in air 
of underground coal mines should be mea- 
sured by the use of an oxygen detector 
with certain properties: 

1. Certified by the Mine Safety and 
Health Administration (MSHA) (i.e., per- 
missible) for use in combustible atmos- 
pheres such as methane in air. 

2. Have a range covering the concen- 
trations encountered in practice (0 to 
25 pet O2). 

3. Meet human related factors — size 
and weight considerations, readability, 
ruggedness . 

4. Meet certain instrument parameter 
values — response time, accuracy, preci- 
sion, and stability. 

5. Meet environmental instrument 
parameters — temperature, pressure, humid- 
ity, and electrical and chemical 
interference. 

The measurement of some of the oxygen 
sensor properties noted in items 4 and 5 
are described in this Bureau of Mines 
report. 

Many commercial oxygen detectors have 
sensors that operate as a galvanic cell. 
The cell anode is a reactive metal such 
as zinc or lead. The cell cathode is a 
noncomsumable metal or graphite structure 
catalyzed with silver to promote oxygen 
reduction. The cell electrolyte is usu- 
ally a potassium hydroxide solution in 
water. Oxygen travels to the cell cath- 
ode through a porous membrane. The po- 
rous membrane limits the mass transport 
of oxygen from the air sample to the cell 
cathode and results in a sensor response 



that is proportional to the oxygen par- 
tial pressure. For a given oxygen volume 
concentration, the oxygen response of 
these cells, because of the membrane- 
diffusion-limited system, depends direct- 
ly on oxygen partial pressure (and hence, 
an ambient pressure) and also is strongly 
dependent on ambient temperature. Detec- 
tors using such cells must use an elec- 
tronic circuit that senses cell tempera- 
ture and corrects for the temperature ef- 
fect of the sensor output. Usually no 
pressure corrections are made electron- 
ically. The detector output is usually 
set to read 21 pet in air known to be 
fresh. From then on altitude or baro- 
metric pressure changes will affect the 
reading even if the actual oxygen concen- 
tration remains at 21 pet. Tests of per- 
formance of some of these oxygen detec- 
tors were reported.^ 

A galvanic oxygen sensor has been de- 
veloped in England that uses a different 
principle for limiting the diffusion of 
oxygen to the cathode — that of diffusion 
through a capillary. This sensor, the 
OTOX model CTL,^ developed at the City 
University, Chemistry Department, St. 
John St., London, was reported to have no 
dependence on atmospheric pressure and 
small dependence on temperature. The 
evaluation of this oxygen sensor was un- 
dertaken in the quest for improved oxygen 
detectors for mining use, and five OTOX 
model CTL oxygen sensors were obtained 
from the National Coal Board for study. 

The sensor (fig. 1) is assembled in a 
cylindrical case, 43 mm high by 23 mm in 
diameter (C flashlight cell size). Al- 
though the mass transport of oxygen must 
pass through an air gap and two mem- 
branes, it is primarily limited by 

'*Ray, R. M. , and F. B. Armstrong. An 
Evaluation of Several Direct Reading 
Electrochemical Oxygen Meters. Bartles- 
ville Energy Research Center (BERC) 
RI 76/7, 1976, 50 pp. 

^Reference to specific products does 
not imply endorsement by the Bureau of 
Mines. 



diffusion through a fine capillary. The in the air at the sensor and is approxi- 
current produced by the cell is proper- mately 1 ma at 20.9 pet O2 . 
tional to the oxygen volume concentration 

THEORY 



The OTOX sensor operates by the gas- 
diffusion-limited mass transport of oxy- 
gen, and the sensor current is determined 
by the overall movement of oxygen as 
follows: 

1 . Oxygen molecules move to the sensor 
face. 

2. Oxygen molecules diffuse through 
the capillary into the sensor interior. 

3. Oxygen within the sensor reacts at 
the cathode surface. 

The mass transport of oxygen (S, g- 
mole/sec) is limited by diffusion through 

TOP VIEW 





To voltmeter 

47-ohm 
resistor 

To voltmeter 



the cell capillary. This flow will fol- 
low Pick's law for steady state diffusion 
in a single dimension and can be written 
as follows: 



s=^o. 



(1) 



where D is the diffusion coefficient, 
cm^/sec, 



A is the surface 



area. 



cm' 



and 



through which diffusion occurs, 

L is the diffusion zone thickness, 
cm, 

C is the ambient oxygen concentra- 
tion, g-mole/cm^. 



This equation assumes a rapid reaction 
of oxygen at the cell cathode. The cur- 
rent in amperes, i, from the galvanic 
cell with limiting cathode reaction is 
related to the oxygen transport rate, S, 
as follows: 



i = n F S, 



(2) 



where n is the number of g equivalents 
per g-mole of reaction (for 
oxygen n = 4) , 

and F is the Faraday constant, 96,480 
amp-sec/g equivalent. 

The molar concentration (C, g-mole/cm-^) 
is related to C , the oxygen concentra- 
tion in percent, as follows: 



C = C 10"2 p/RT, 

where P is the pressure, atm, 

R is the gas constant, 
82.06 atm cm^ , 



(3) 



FIGURE 1. - OTOX oxygen sensor. 



and 



mole K 
T is the absolute temperature, K. 



By combining the three 
cell current, i, is equal 
ing expression: 



equations, the 
to the follow- 



^ 100 L R T 



(4) 



temperature and pressure becomes equal to 
the following expression: 



i = K' t1/2 c«, 
where K' is a constant. 



(6) 



With the cell 
rameters fixed, 
written: 



capillary physical pa- 
the equation may be 



= Kf C. 



where K is a constant. 



(5) 



The gas diffusion coefficient (D, cm^/ 
sec) is a function of the average gas 
molecule velocity and the mean free path 
of the gas molecule. In free space it is 
a function of T^^^ P~ ' , and thus the cell 
current i expressed as a function of 



The equations for gas diffusion are ap- 
plicable to this system since the OTOX 
cell has a capillary with a size much 
greater than the oxygen mean free path. 
When the oxygen reaching the cathode is 
limited by gas diffusion rather than mem- 
brane diffusion, the cell current is a 
function only of the square root of the 
absolute temperature, and not of pres- 
sure. Thus, the OTOX sensor current 
should be directly proportional to 
the oxygen percent concentration and not 
to oxygen partial pressure as in many 
other oxygen sensors that have been 
investigated. 



EXPERIMENTAL SYSTEMS 



A load resistor was connected to the 
sensor leads, and the OTOX sensor current 
was measured by reading the voltage 
across the load resistor using a Keithley 
model 172 DMM digital multimeter. A 147- 
ohm resistor was chosen for the load 
since this value of resistance yielded 
maximum sensor power output and matched 
the internal sensor resistance. Room air 
(20.9 pet O2) was used at local tempera- 
tures and pressures for stability test- , 
ing. An Aminco environmental chamber was 
used for controlled-temperature experi- 
ments. A Boekel chamber was used for 
controlled-pressure measurement, with 
differential pressure readings taken from 
a mercury manometer. Oxygen and nitrogen 
from cylinders were mixed to form concen- 
trations of oxygen above 20.9 pet. Cyl- 
inder nitrogen was used to dilute air to 
form oxygen concentrations below 20.9 
pet. A Taylor Servomex model A0272 oxy- 
gen analyzer and an Edmont-Wilson amper- 
ometric membrane sensor both were used to 
measure the oxygen concentrations of the 
prepared air mixtures. 



A Hewlett-Packard model 7100 strip 
chart recorder was used to record sensor 
response for subsequent measurement of 
response times, as follows: A sensor, 
initially in air, was plunged into nitro- 
gen gas flowing at 50 mL/min by placing 
over the sensor a polyethylene cylinder 
connected by tubing to a nitrogen cylin- 
der. The nitrogen flowed past the sensor 
and exited through 1/8-in-diameter holes 
in the cylinder to prevent pressure 
increase at the sensor. The response 
time for the sensor for a step decrease 
in oxygen was measured from the strip 
chart recording as the time from the ini- 
tial change in response to 90 pet of 
the final response. By flowing air from 
a cylinder through a similar fitting, the 
response time for a step increase in oxy- 
gen was determined. In this procedure 
the cell, initially in nitrogen, was 
rapidly immersed in a flowing air atmos- 
phere (20.9 pet O2), and the cell current 
was recorded. 



DISCUSSION 



STABILITY 

Five OTOX sensors were tested in room 
air (20.9 pet O2) for a 13-day period 
during which the millivolt output of the 
sensors was monitored under a continuous 
147-ohm load. The room temperatures and 
pressures varied during the test. The 
data obtained are summarized below, in 
millivolts : 



Sensor 


Initial 


Average 


Std. dev. 


1 


146.0 


145.5 


0.5 


2 


162.0 


163.3 


1.0 


3 


146.0 


144.9 


.4 


4 


151.0 


154.0 


1.6 


5 


163.0 


164.6 


1.4 



The data were taken over 13 days with 
fluctuations of temperature from 18° to 
24° C. The maximum coefficient of varia- 
tion (the standard deviation divided by 
the average value) is 1.04 pet. The mea- 
sured values are plotted in figure 2. 
The data indicated no general increase or 
decrease of the values of the sensor re- 
sponse to 20.9 pet O2 for this brief 
test. 



The OTOX sensor stability was also mea- 
sured over a 146-day (4.7-month) period. 
These data were analyzed to obtain infor- 
mation on the drift or long-term change 
in sensor output. The drift is expressed 
as a linear function of time using a 
least squares regression analysis. The 
precision of the data can be estimated 
from the standard deviation of the ob- 
served data about the calculated drift 
line. The OTOX sensor outputs for 20.9 
pet O2 and room temperature (19° through 
23° C) are summarized in table 1 , and the 
data are plotted in figure 3. Table 2 
summarizes the equation parameters for a 
least squares regression analysis. 

It is apparent from both the graph and 
the calculated slopes of the stability 
data that sensor 1 is significantly dif- 
ferent over long time observations from 
the other sensors. The measured output 
of sensor 1 in room air shows almost a 
ten-fold greater negative drift than that 
of the other sensors. The average re- 
sponse of the other sensors gave little 
variation over the 4.7-month period. The 
average of the slopes of the curves for 



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FIGURE 2. 



6 8 

TIME, days 

OTOX sensor initial stability. 



10 



14 



TABLE 1. - Stability of OTOX sensors; sensor response 
for 20.9 pet O2 , raV 



Day 


No. 1 


No. 2 


No. 3 


No. 4 


No. 5 





146.34 


161.70 


146.18 


151.46 


162.75 


6 


144.92 


162.89 


144.57 


153.82 


164.74 


7 


145.57 


164.01 


144.93 


154.50 


165.98 


12 


144.90 


165.13 


145.50 


156.23 


166.53 


13 


144.82 


162.14 


143.28 


154.03 


163.77 


39 


139.56 


164.21 


144.49 


153.62 


165.04 


49 


139.30 


163.92 


144.70 


153.24 


166.04 


84 


132.56 


163.94 


144.61 


153.27 


165.52 


88 


132.46 


166.30 


146.46 


155.73 


167.28 


98 


135.76 


164.33 


146.90 


155.76 


165.48 


101 


136.48 


164.16 


147.96 


152.33 


162.63 


103 


136.83 


163.45 


144.69 


152.31 


164.75 


104 


137.63 


16a.92 


145.33 


156.32 


164.63 


Ill 


134.39 


163.97 


147.34 


157.24 


164.88 


115 


133.71 


165.37 


147.47 


156.26 


164.79 


116 


134.38 


164.59 


146.58 


152.76 


165.45 


118 


136.66 


164.32 


148.14 


152.73 


166.49 


119 


134.49 


165.63 


148.08 


153.69 


165.38 


124 


135.35 


164.17 


146.36 


153.60 


165.52 


125 


134.51 


164.04 


146.34 


152.86 


165.00 


126 


133.60 


163.43 


145.69 


152.36 


164.29 


130 


133.99 


163.88 


145.99 


151.50 


164.82 


131 


131.65 


162.57 


145.18 


151.02 


164.12 


136 


129.42 


163.61 


146.24 


151.77 


164.63 


138 


129.35 


163.18 


146.16 


150.98 


164.44 


142 


129.44 


163.89 


146.30 


152.28 


164.74 


144 


126.61 


163.94 


146.73 


152.36 


165.19 


146 


126.62 


164.38 


147.02 


152.21 


165.22 



TABLE 2. - Linear regression analysis for stability data on OTOX sensors^ 





Least squares analysis 


Squared 
regression 
coefficient, raV 


Standard deviation, mV 


Sensor 


Slope, mVAday 


Intercept, raV 


Slope 


Regression 


1 


-0.10700 

.00525 

.01430 

-.01080 

.00009 


145.8 
163.5 
144.7 
154.5 
165.0 


0.85200 
.06740 
.31900 
.08800 
.00002 


0.0088 
.0038 
.0041 
.0068 
.0041 


2. 18 


2 


.96 


3 

4 

5 


1.02 
1.70 
1.03 







Equation response (mV) = slope x elapsed time + intercept, 



sensors 2 through 5 is nearly zero. Lim- 
its of the values for true slope 3 may be 
estimated at a given risk a (say, a = 5 



pet) from the experimental data for cal- 
culated slope b from sample data as 
follows: 



'yx 



< 3 < b + t 



'yx 



170 



160 



> 

E 

{^'150 

z 

o 

a. 

</) 

UJ 

cr 



140- 



130- 



120 



1 1 1 1 


- 


— ^^7^ 


^^ 


- 


- 






- 


^ 




~'^^:; 


- 


- 


1 1 


1 1 


-.- 



40 



80 120 

TIME, days 



160 



200 



FIGURE 3. - OTOX sensor long-term stability. 

where t is the value taken from the Stu- 
dent t tables at the n-2 (26) sample lev- 

el, where a = 0.05 and the ratio ( tt — 1 is 



'" ( sf ) '■ 



the standard error of the slope with val- 
ues given in table 2. The following es- 
timates for the limits of values of the 
true slope 3 are obtained: 



Sensor 


3 limits 




Upper 


Lower 


1 


-0.860 
.014 
.024 
.0054 
.0098 


-0.128 


2 


-.0038 


3 


.0045 


4 


-.027 


5 


-.0097 



From data on measurements taken on sen- 
sors 2, 4, and 5, the true slope could be 
zero. For sensors 1 and 3 the data taken 
do not support the hypothesis that the 
true slope is zero; however, the slope of 
sensor 3 is small. 

The square of the regression coeffi- 
cient , a measure of the dependence of the 
sensor response on time, was large for 
sensor 1 but approximated zero for the 
other sensors. The coefficient of varia- 
tion for the regression line or the 



scatter of the data about the calculated 
line ranged from 0.6 to 1.5 pet for the 
five sensors. 

The original response data and the cal- 
culated intercepts show that the sensors 
are not identical and could not be 
directly interchanged in a commercial 
oxygen detector without electrical 
compensation for sensitivity and zero 
offset. This nonuniform response may be 
due to variation in construction and dif- 
ferences in dimensions of materials used 
in the sensor. 

BIAS OVER RANGE 

The response of the OTOX sensors, 
placed in an environmental chamber, was 
measured while the atmosphere was changed 
in steps from to 40 pet O2. The mea- 
sured sensor responses are listed in ta- 
ble 3 for different oxygen concentra- 
tions. Since the sensor responses were 
not zero in nitrogen, or pet O2 , the 
data were corrected by subtracting the 

measured response in nitrogen from the 
measurements taken at the other oxygen 
concentrations. A graph made from this 
data is given in figure 4. 

The data are linear for lower values of 
oxygen and deviate from a straight line 
above 35 pet O2. This nonlinearity of 
the complete data was confirmed by a 
least squares analysis of the data for 
the five sensors. 

The average sensor sensitivity (milli- 
volts per percent oxygen) in normal air 
(20.9 pet O2) follows: 

Sensor 1 6.18 

Sensor 2 7.75 

Sensor 3 6.93 

Sensor 4 7.20 

Sensor 5 7.81 

The sensor response data were normal- 
ized to yield values of indicated percent 
oxygen by dividing the zero corrected re- 
sponse data for each sensor by the corre- 
sponding sensor average response ratio. 



TABLE 3. - OTOX sensor oxygen response, mV 



Oxygen , pet 

O.O.. rrrr 

5.0 

10.0 

15.0 

17.0 

18.0 

20.0 

20.9 

20.9 

22.0 

25.0 

30.0 

35.0 

40.0 

45.0 



No. 1 



1.58 

28.18 

51.24 

85.13 

97.01 

108.42 

117.42 

130.25 

130.87 

130.73 

149.17 

184.21 

222.76 

240.74 

260.64 



No. 2 



1.72 
32.75 
63.15 
105.64 
120.39 
133.87 
145.70 
164.62 
163.11 
162.82 
185.98 
229.03 
270.52 
286.57 
302.20 



No. 3 



1.65 

28.90 

56.09 

94.39 

107.69 

119.24 

130.34 

147.12 

146.16 

147.28 

167.24 

207.73 

252.79 

272.48 

292.30 



No. 4 



1.58 

32.12 

59.77 

99.99 

113.21 

126.51 

137.31 

152.84 

151.35 

152.45 

174.30 

211.99 

248.93 

263.25 

277.84 



No. 5 



1.73 
31.76 
63.49 
106.60 
121.54 
134.41 
146.96 
165.58 
164.42 
166.04 
188.27 
232.54 
277.23 
294.36 
311.40 



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350 



300- 



250- 



200- 




15 20 25 30 

ACTUAL OXYGEN, pet 



35 



40 



45 



FIGURE 4. - Zero-corrected oxygen response for the OTOX sensor. 



Table 4 summarizes this normalized sensor 
response data, giving the actual percent 
oxygen as determined by the reference 
instruments and the indicated percent 
oxygen values for each sensor. Figure 5 
is a graph of the normalized responses of 
the sensors for varying oxygen concentra- 
tions to 30 pet. Figure 5 also contains 
the line calculated from the least 
squares analysis of the data. Table 5 
summarizes the values of the slope, in- 
tercept, and standard deviation for the 
derived curves. The slopes of the calcu- 
lated curves are close to 1.0, indicating 
equivalency of the actual and indicated 
percent oxygen values. The excellent fit 
of the experimental data to a straight 
line is indicated by the near-unity value 
for the square of the regression 
coefficient. 

TABLE 4. - OTOX sensor normalized 
oxygen response data, pet 




10 15 20 
ACTUAL OXYGEN, pet 

FIGURE 5. - Normalized oxygen response for 
the OTOX sensor. 



Oxy- 












gen, 


No. 1 


No. 2 


No. 3 


No. 4 


No. 5 


pet 












0.0. . 


0.00 


0.00 


0.00 


0.00 


0.00 


5.0. . 


4.30 


4.00 


3.93 


4.24 


3.84 


10.0.. 


8.03 


7.92 


7.85 


8.08 


7.90 


15.0.. 


13.51 


13.40 


13.38 


13.66 


13.42 


17.0.. 


15.44 


15.31 


15.30 


15.50 


15.34 


18.0.. 


17.28 


17.05 


16.96 


17.35 


16.98 


20.0.. 


18.74 


18.57 


18.57 


18.85 


18.59 


20.9.. 


20.82 


21.01 


20.99 


21.00 


20.97 


20.9.. 


20.92 


20.82 


20.85 


20.80 


20.83 


22.0.. 


20.89 


20.78 


21.01 


20.95 


21.03 


25.0.. 


23.88 


23.77 


23.89 


23.98 


23.88 


30.0.. 


29.55 


29.33 


29.73 


29.22 


29.55 



TEMPERATURE EFFECT 

The OTOX sensors were placed in the en- 
vironmental chamber, and the temperature 
was varied in steps from 5° to 40° C. 
The sensor responses were measured while 
in air (20.9 pet O2) at the different 
temperatures and were corrected to per- 
cent oxygen readings by dividing by the 
measured responses value (aiillivolts) at 
22° C and multiplying by 20.9 pet. Ta- 
ble 6 lists the data obtained, and fig- 
ure 6 shows the data. Sensor 1 data are 
not included in the figure since the 
curve was not the same as for the other 



TABLE 5. - Linear regression analysis for OTOX response to oxygen 



concentration 



Sensor 



Least squares analysis 



Slope' 



Intercept, pet 



Squared 
regression 

coefficient , 
pet 



Standard deviation, pet 



Slope 



Regression 



0.9998 
.9998 

1.0109 
.9969 

1.0086 



-0.864 
-.979 

-1.126 
-.791 

-1.097 



0.994 
.993 
.993 
.995 
.993 



0.0245 
.0264 
.0274 
.0231 
.0263 



Normalized response 
^ pct oxygen reading ^ 
pet gaseous oxygen 



= slope X true oxygen pet + intercept, 



0.687 
.739 
.767 
.648 
.735 



10 



TABLE 6. - OTOX sensor temperature response 
in air, pet 



Temperature, ° C 


No. 1 


No. 2 


No. 3 


No. 4 


No. 5 


5 


18.66 


19.53 


19.88 


18.49 


19.56 


10 


20.30 


20.26 


20.50 


19.65 


20.22 


15 


20.83 
20.83 


20.57 
20.74 


20.81 
20.93 


20.25 
20.61 


20.59 


20 


20.74 


22 


20.90 
20.30 


20.90 
21.02 


20.90 
21.13 


20.90 
21.08 


20.90 


25 


20.98 


30 


19.45 


21.21 


21.30 


21.40 


21.17 


35 


19.12 


21.32 


21.36 


21.62 


21.26 


40 


19.04 


21.25 


21.23 


21.65 


21.20 



four sensors. The maximum changes for 
the readings are 2 pet of oxygen response 
increase for a 35° C temperature 
increase. 

If the principal mechanism limiting the 
transport of oxygen is by diffusion 
through a fine capillary, then, as indi- 
cated in equation 6, a plot of the square 
root of the absolute temperature versus 
sensor response should yield a straight 
line. Figure 7 shows such a plot. This 
relationship gives a curve that is appar- 
ently no more linear than that of fig- 
ure 6. The curvature in this plot could 



be due to the diffusion of oxygen being 
limited, by transport through a membrane 
as well as by the capillary. Upon dis- 
assembly of a sensor, we found two Teflon 
membranes within the cell separating the 
capillary from the cathode, which are 
used with the capillary and dead air 
space to form the total diffusion trans- 
port system for the oxygen sensor. 

PRESSURE EFFECT 

The sensors' response to change of air 
pressure was measured with ambient air 
(20.9 pet O2) at an average pressure of 




20 

TEMPERATURE, **C 

FIGURE 6. - OTOX sensor temperature response. 



11 



22.0 




I8.5L-X- 
16.6 



17.8 



\/T, CK)^ 



FIGURE 7. - OTOX sensor temperature response 
for the square root of the absolute temperature. 



97.923 X 10^ Pa (28.92 in Hg). Table 7 
lists the measured response changes, and 
figure 8 plots the responses over a range 
of air pressures. Measurements were also 
taken of the pressure response of the Ed- 
mont Wilson membrane-amperometric oxygen 
analyzer (membrane-limited transport) and 
of the Taylor Servomex paramagnetic oxy- 
gen analyzer, model A0272, used as refer- 
ence analyzers. Each of the reference 



analyzers has a pressure response of 
3.46 pet of reading per 33.864 x lO^ pa 
( 1 in Hg) change in pressure. The pres- 
sure unit of 1 in Hg was chosen since 
this is equivalent to 1,000 ft of alti- 
tude change. The -3.46-pct change in 
response would also be found on trans- 
porting either of the reference oxygen 
analyzers from sea level to a 1,000-ft 
elevation. 

For sensors with a membrane-limited 
transport system, the sensor response to 
pressure changes was given previously in 
equation 5. For gas diffusion through a 
membrane, the gas diffusion coefficient D 
is proportional to T^'^ and not dependent 
on pressure P; thus, by substitution for 
D in equation 5 we obtain 



i = K' T" 



1/2 



PC 



(7) 



or 



i = K' PC for constant 

temperature, (8) 



where, again, i is the sensor response 

current , amp , 



and 



P is the pressure, atm, 

C is the oxygen concen- 
tration, pet. 



The change in response Ai for a change in 
pressure AP is 



Ai = KAP C , 



(9) 



TABLE 7. - Effect of static pressure on sensor response, 
OTOX CTL, pet 



Air pressure 
change, in Hg 


No. 1 


No. 2 


No. 3 


No. 4 


No. 5 


ED^ 


Ti^ 


-8 

-6 

-4 

-2 


19.90 
20.12 
20.33 
20.52 
20.53 
20.90 
21.02 
21.16 
21.31 


20.54 
20.69 
20.84 
20.93 
21.08 
21.11 
21.18 
21.29 
21.38 


20.26 
20.46 
20.70 
20.92 
21.09 
21.23 
21.31 
21.49 
21.58 


20.09 
20.50 
20.56 
20.79 
20.99 
21.15 
21.28 
21.42 
21.57 


20.41 
20.58 
20.73 
20.77 
20.91 
21.05 
21.12 
21.17 
21.22 


15.1 

16.7 

18.2 

19.4 

20.9 

22.5 

24.0 

25+ 

0) 


15.2 
16.7 
18.1 
19.5 





20.9 


2 


22.3 


4 


23.8 


6 


25.5 


8 


0) 


^Edmont Wilson 
^Taylor Servon 
^Off scale. 


L oxygen 
lex A027. 


sensor. 
I oxygen 


analyze! 


r. 









12 




Edmont Wilson galvanometric-membrane 

Taylor paramagnetic 

OTOX oxygen sensor-capillary 



1 



-8 



-6 



4-2024 
MERCURY PRESSURE CHANGE, in 

FIGURE 8. - OTOX sensor pressure effect. 



8 



and the relative percent change in re- 
sponse is obtained by dividing equation 9 
by equation 8 and multiplying by 100 as 
follows : 



^1 inn ^ ^P 
-: — • 100 pet = Tjr— 

1 ^ P 



100 pet. (10) 



Thus, the calculated relative percent 
change in percent sensor response is 
equal to the percent pressure change. A 
pressure change of 33.864 x 10^ Pa (1 in 
Hg decrease) is a 3.46-pct change at our 
test elevation of 1,000 ft above sea 
level. 

The paramagnetic analyzer does not rely 
on oxygen diffusion, and the paramagnetic 
sensor response is simply proportional to 
the oxygen concentration (gram-mole per 
unit volume) and thus proportional to 
oxygen pressure. The calculated sensor 
response change per inch of mercury 



pressure for the paramagnetic sensor is 
3.46 pet, the same as calculated in equa- 
tion 10. The measured sensor response 
change per unit pressure of both refer- 
ence instruments was approximately equal 
to this calculated value. On the other 
hand, the OTOX sensors have a pressure 
response change of only 0.34 pet per 
33.864 X io2 Pa (1 in Hg). This is 1/10 
of the change for the reference units, 
and the deviation of this number from 
zero again may be due to the presence of 
the two mechanisms for diffusion control 
used in the OTOX sensor, that of a capil- 
lary air space and the membranes within 
the sensor. 

Table 8 summarizes the results of the 
statistical tests for the effect of pres- 
sure on the sensor response. The values 
for the square of the regression coeffi- 
cient approach unity and show the excel- 
lent fit for a linear dependence of 



13 



TABLE 8. - Linear regression analysis for OTOX response to change 
in static pressures' 





Least squares analysis 


Squared regression 
coefficient, pet 


Standard deviation 


Sensor 


Slope, 


Intercept, 


of regression, pet 




pct/in Hg 


pet 








1 


0.088 


20.64 


0.99 




0.06 


2 


.05 


21.00 


.98 




.05 


3 


.08 


21.00 


.98 




.07 


4 


.09 


20.93 


.98 




.08 


5 


.05 


20.89 


.97 




.11 


Taylor 












Servomex 


.73 


21.01 


1.00 




.04 


Edmont 












Wilson. . 


.71 


20.90 


1.00 




.04 



Equation 



Indicated O2 , pct = slope x Ap inch Hg + intercept. 



response on pressure. The OTOX sensors 
give oxygen response readings propor- 
tional to the oxygen concentration rather 
than to the oxygen partial pressure as do 
the reference detectors. This response 
to gas concentrations is unique since all 
other electrochemical sensors, and almost 
all other sensors using infrared absorp- 
tion or refractive index measurements, 
respond to partial pressure of the sensed 
gas. The ANDROS handheld CO2 or CH4 de- 
tector using a pressure modulated infra- 
red absorption technique is the only 
other sensor we have found with the abil- 
ity to read the concentration of gas 
directly.^ 

PRESSURE SURGE TESTS 

The OTOX sensor contains free space 
or dead volume within the cell interior 
between the end of the capillary and the 
membranes . The oxygen in this volume 
must be reacted before equilibrium oxy- 
gen concentrations and stable sensor re- 
sponse values can be obtained. Pressure 
surges up to 24.90 x 10^ Pa (10-in water 
pressure) may occur when a miner moves 
through airlocks (sealed doors) separat- 
ing aircourses in underground mines. 
Figure 9 shows a plot of data obtained 

"Chilton, J. E., C. R. Carpenter, and 
G. H. Schnakenberg. Improvements in Gas 
Detector Instrumentation. Proc. 5th WVU 
Conf. on Coal Mine Electrotechnology , 
Dept. of Elec. Eng. , W. Va. Univ., Mor- 
gantown, W. Va., July 30-Aug. 1, 1980, 
pp. 19-1 to 19-17. 



from the response measurements for a sen- 
sor encountering a pressure change from 
+270.91 X lo2 Pa (8 in Hg) to atmospheric 
pressure. The initial response change is 
due to reduction of oxygen quantity in 
the capillary by the outward flow of 
oxygen-def f icient from the space in the 
cell. Equilibrium diffusion and a stable 
response are reestablished at atmospheric 
pressure in 30 sec (time to reach 90 pct 
of final value). 

Figure 10 shows the reponse change for 
an air pressure change from -270.91 x 10^ 




20 



60 



80 



40 
TIME, sec 

FIGURE 9. - OTOX sensor pressure surge ef- 
fect: Initial pressure +8 in Hy above atmospheric 
pressure. 



14 



240 



120 




1/21.1 pet O2 







20 



40 
TIME, sec 



60 



80 



FIGURE 10. ■■ OTOX sensor pressure surge 
effect: Initial pressure -8 in Hg below atmos- 
pheric pressure. 

Pa (-8 in Hg) pressure to atmospheric 
pressure. The increase in cell response 
is due to the oxygen in air flowing into 
the sensor internal free space, and the 
equilibrium sensor response is reestab- 
lished in 60 sec (time to obtain 90 pet 
of final value). These experimental 
pressure surges are greater than would be 
found in operating mines, and response to 

upset from smaller pressure surges would 
more quickly return to the original val- 
ue. Table 9 summarizes the times to re- 
cover obtained by the pressure surge 
test. The average time to recover to 90 
pet of original response value is 22 sec 
for a negative pressure surge; full re- 
covery occurs in an average of 55 sec. 
The average time to recover to 90 pet of 
the original response value is 61 sec for 
positive pressure surges; full recovery 
is in an average of 98 sec. The measured 
times depend both on quantity of oxygen 
to be consumed, which is a measure of 



sensor free space volume relative to the 
surge volume through the capillary, and 
on the sensor current, which controls the 
rate of oxygen consumption. This current 
is set by the value of the load resistor 
placed across the sensor. 

TABLE 9. - Pressure surge response 
time data — time in seconds for 
recovery of response to 90 pet 
of initial value 





Sensor 


Pressure change 




Negative 
(+8 in Hg 
to in Hg) 


Positive 
(-8 in Hg 
to in Hg) 


1 


20 
16 
29 
27 
21 


45 


2 


62 


3 


52 


4 


80 


5 


65 



RESPONSE TIMES 

Response times were measured for a step 
decrease in oxygen by first equilibrating 
the sensor in air and then flooding it 
with flowing nitrogen at constant ambient 
pressure. Response times were measured 
also for a step increase in oxygen con- 
centration by equilibrating the sensor in 
nitrogen and then flooding it with flow- 
ing air at ambient pressure. Figure 11 
shows the sensor response change for both 
conditions: The step increase and step 
decrease in oxygen. Table 10 reports the 

TABLE 10. - Response times for step 
change in oxygen concentration to 
90 pet of final value, sec 





Sensor 


Test sequence 




Step decrease 
in oxygen 


Step increase 
in oxygen 


1 


70 
50 
26 
28 
45 


76 


2 


28 


3 


32 


4 


32 


5 


25 



'For a step decrease, the sensor is 
originally in air (20.9 pet O2). then im- 
mersed in nitrogen (0 pet O2); for a step 

-ir»or-£a<aeci ♦•V»£i c £^-r%c r\ir 2.S OlTi 21113.11 V Itl 



mersed in nitrogen 

increase, the sensor _ 

nitrogen, then immersed in air (20.9 pet 

O2). 



15 




60 80 

TIME, sec 

FIGURE 11 . = Sample time responses for the OTOX sensor. 



60 



response times in seconds for both condi- 
tions as times for response to change to 
90 pet of the final value. The average 
values of 44 sec for a step decrease and 
39 sec for a step increase in oxygen con- 
centration were measured. As noted in 
the last section, the response times are 
functions of sensor internal volumes and 
of the sensor currents generated by reac- 
tion of the oxygen. The sensor current 
is a function of sensor load resistance 



value. This current is maximized, and 
the response time is shortened by the use 
of smaller load resistors. However, sen- 
sor life is shortened by using small 
resistors as loads since the anode con- 
sumption is proportional to sensor out- 
put current. An operational amplifier 
coupled with the sensor to form a 
current-to-voltage converter can also 
yield minimum response times for this 
type of oxygen detector. 



SUMMARY 



a unique 
The lim- 



The OTOX model CTL sensor is 
electrochemical oxygen sensor, 
iting oxygen mass transport process is 
the diffusion of oxygen through a small- 
diameter capillary. Oxygen is rapidly 
consumed at an inert cathode, and the 
complete galvanic cell also contains a 
consumable metal anode with an alkaline 
electrolyte. Because of this design, the 
sensor has a response with a small tem- 
perature coefficient and very small pres- 
sure coefficient. The sensor responds 



to oxygen concentration rather than oxy- 
gen partial pressure because of the mini- 
mum pressure dependence. The stated sen- 
sor life is 6 months in ordinary air 
atmospheres, but operation in high oxygen 
concentrations (greater than 30 pet) , 
with substantial quantities of acid gases 
(HCl or CO2), or in very dry environ- 
ments, and the use of electronic cir- 
cuitry with small load resistances may 
shorten the operating life. A summary of 
the measured parameters follows: 



16 



1. Average response time to 90 pet of 
final value by step change of oxygen con- 
centration is 42 sec. 

2. Average response time to 90 pet of 
initial value by pressure shock of 
+270.91 X 10^ Pa to ambient pressure (+8 
in Hg to ambient pressure) is 61 sec. 

3. Sensor response change to tempera- 
ture for 5° to 40° C range is +2.9 pet 
reading change per 10° C. 

4. Sensor response change to pressure 
change in the range ±270.91 x 10^ Pa (+8 



in Hg) is +0.34 pet reading change per 
33.864 X 102 Pa (1 in Hg or 1,000 ft of 
altitude change). 



5. Range is linear from to 30 pet 
oxygen; the 95-pct confidence limit for 
indicated percent oxygen at 20.9 pet O2 
is ±2.84 pet of reading. 

6. Interehangeability of sensors: The 
OTOX sensors have significantly different 
output. Sensor replacement with recali- 
bration would be performed by gain and 
zero offset adjustment of the detector. 



<'U.S. GOVERNMENT PRINTING OFFICE: 1983-605-015/20 



INT.-BU.OF MINES, PGH., PA. 26659 




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